Disk brakes in motor vehicles exhibit brake pads which, depending on an actuation of a braking encoder, are pressed by a pressure spindle onto a brake disk connected to a wheel to be decelerated, in order to be able to transmit a braking action to the brake disk and the wheel. Brake pads of such a type wear out in the course of use of the disk brake—that is to say, their thickness decreases. As a result, a braking stroke of the brake pads—that is, the distance that the brake pads travel from a disengaged position, in which the braking encoder has not been actuated, into a braking position upon actuation of the braking encoder in order to obtain a braking action—is lengthened.
In order to obtain a rapid braking action with a short braking stroke also in the event of wear, an adjusting device—for instance, an adjusting gear mechanism—is provided, with which the decreasing thickness of the brake pads can be compensated, by the brake pads being repositioned closer to the brake disk again. As a result, roughly the original braking stroke can be set which is present in the case of new brake pads. If the brake pad is exchanged in the event of a certain wear, the adjusting device is reset—for instance, by a resetting tool—into the original starting position again, in order to re-adapt the braking stroke to the new pads.
In order also to be able to measure the wear, a monitoring device is provided which is operatively connected to the adjusting device in such a way that it registers an adjustment distance of the adjusting device, from which the wear follows directly.
EP 1 538 364 B1 discloses for this purpose a disk brake in which the pressure spindle with the brake pads can be brought closer to the brake disk by an adjusting gear mechanism, by the pressure spindle being unscrewed from a nut with increasing wear. In order to measure the wear and also the braking stroke, a wear-monitoring device is provided which is constituted by a longitudinally movable, spring-preloaded pin which is supported on a front side of the moving pressure spindle and is secured to the brake caliper of the disk brake in such a way that a movement of the pressure spindle is transmitted to the pin under spring loading.
If the pressure spindle is unscrewed from the nut for the purpose of compensating for the wear, the pin also moves relative to the brake caliper, and this can be registered optically or electrically. Furthermore, the pin is repositioned also in the event of an actuation of the brake, in the course of which the pressure spindle moves in the direction toward the brake disk (braking stroke). Consequently, relative displacements that result from the wear or from the braking stroke can be registered by the pin directly and in unfalsified manner. The registered relative displacements can be detected by a potentiometer, and the signals thereof can be processed further via an electrical connection.
A disadvantageous aspect of this solution is that the potentiometer of the wear-monitoring device for electrical registration of the movement of the pin has a limited service life, so that an exchange of the potentiometer may occur during the lifespan of the disk brake.
WO 2013124247 A1 presents a monitoring device for measuring an adjustment distance of an adjusting device. Accordingly, a gear wheel, which is seated on a shaft, meshes with the pressure spindle of the adjusting device, so that in the event of a rotation of the pressure spindle a rotation of the shaft is brought about. The rotation of the shaft can be registered by a monitoring device. For this purpose the monitoring device exhibits a portion with wedges and grooves, via which the rotation of the shaft is transmitted to a hollow shaft. The rotation of the hollow shaft is transmitted by a reduction gear to a rotary body. The rotary body takes the form of a two-pole magnet, the rotation of which is registered in contactless manner by a Hall sensor and can be converted into the corresponding adjustment distance or wear.
A disadvantageous aspect in this case is that very many components—in particular, wearing gear wheels for the reduction gear—are required in order to register the wear. In addition, the monitoring device is provided as an external device to be mounted and fastened on the housing of the brake caliper, as a result of which additional construction space on the brake caliper is needed. In addition, the braking stroke cannot be registered by the monitoring device in the event of actuation of the disk brake, since the adjusting device does not rotate in the course of braking.
A reset is guaranteed by an additional resetting tool which has to be inserted through the monitoring unit upon each reset in order to rotate the pressure spindle into its original starting position again; consequently a reset is elaborate, and a special tool is needed.
EP 1 633 992 B1 presents, furthermore, a monitoring device for monitoring the wear of a brake pad of disk brakes. Accordingly, a first sensor and a second sensor, taking the form of a monitoring device, are provided, the first sensor registering whether a deceleration obtains, and, depending thereon, the second sensor measuring the movement of the adjusting device in contactless manner.
For this purpose, inside the adjusting device a magnetic measuring element taking the form of a tube is provided which is fixed with respect to the brake pad and arranged around a Hall sensor in linearly repositionable manner. The length of the magnetic tube corresponds approximately to a maximally possible repositioning distance by which the brake pad can be repositioned in the course of braking (braking stroke) or by the adjusting device (adjustment distance). In contrast, the Hall sensor is constructed to be smaller, so that, depending on the position of the brake pad, the Hall sensor enters the tube to a greater or lesser depth and, as a result, depending upon the depth of entry into the tube, a corresponding signal is generated by the Hall sensor, from which the distance traveled by the brake pad can be ascertained.
A disadvantageous aspect in this case is that the measuring element takes the form of an elongated magnetic tube. Making a tube of such a type magnetically homogeneous over a great length is a very elaborate process, so inaccuracies of measurement may occur if the magnetic field within the magnetic tube varies. In addition, introducing the elongated tube into the adjusting device is an elaborate process, so that exchange or retrofitting is associated with high assembly costs. Aligning the magnetic tube in the adjusting device with respect to the Hall sensor is also difficult.
Furthermore, for the purpose of identifying whether the output signal is specifying the braking stroke or the adjustment distance an additional signal transmission is necessary which specifies, depending on the result ascertained by the first sensor, whether or not a deceleration obtains—that is to say, whether a braking stroke or an adjustment distance is being output. Only with this additional signal can the downstream electronics decide whether a braking stroke or a wear is to be calculated.